Styrene resins are in wide use as various molding materials because they show an excellent balance between price, mechanical strength and moldability. The mechanical strength and the moldability depend upon an average molecular weight of the styrene resin. Styrene resins with a higher average molecular weight have a better mechanical strength, and those with a lower average molecular weight have a better moldability.
Therefore, styrene resins with various average molecular weights must be used properly according to intended uses, and hence the development of a process has been awaited which can produce styrene resins having an intended average molecular weight with low cost and with good efficiency. To attain a low cost and efficient production, a polymerization rate must be controlled carefully according to the situation of demand. Moreover, it is necessary that when an equipment trouble happens to occur the polymerization rate is rapidly decreased without changing the average molecular weight of the product and that when the trouble has been removed the polymerization rate is rapidly increased. Accordingly, a method for controlling the polymerization rate easily and in a short time is eagerly desired.
Known processes for industrial production of styrene resins include a thermal polymerization process and an initiator polymerization process.
In the thermal polymerization process, in order to control a polymerization rate without changing an average molecular weight of a resin formed, it is necessary to change a volume of a polymerization solution while maintaining a polymerization temperature constant. However, increasing the volume of the polymerization solution has its limit because of the limitation of dimension of polymerization vessel, and hence the production rate cannot be changed at will. Furthermore, it takes a long time to regulate the volume of the polymerization solution.
In the initiator polymerization process, aside from a method of changing a volume of polymerization solution similar to that described above, a method has been proposed in which an amount of the polymerization initiator fed into a polymerization reactor is changed. In this method, however, a polymerization temperature must be changed simultaneously in order that an average molecular weight of a resin formed may be kept constant. Resultantly, the method has the disadvantage of being unable to control a polymerization rate as desired.
JP-B-41-19511, JP-B-52-42834, JP-B-52-797, JP-B-55-7455 and JP-B-2-21401 disclose methods of increasing a polymerization rate by using a specific polyfunctional organic peroxide as a polymerization initiator.
However, even when these methods are used, it is similarly necessary to change a polymerization temperature, although the controllable range of polymerization rate widens. Therefore, two parameters, that is, the polymerization temperature and the amount of the polymerization initiator charged, must be simultaneously operated to control the polymerization rate and the average molecular weight of styrene resin at the same time; hence the polymerization conditions cannot be easily controlled. Furthermore, since the polymerization initiator used is a specific polyfunctional initiator and other conventional inexpensive polymerization initiators cannot be used, these methods lead to a high production cost.
In view of the situations, the present inventors have made extensive study on the method for controlling a polymerization rate. As the result, the inventors have found a method which can control a polymerization rate easily and in a short time while maintaining an average molecular weight of styrene resin formed constant. The present invention has been attained on the basis of above finding.